What matters in traditional formal verse is the alternation
between (relatively) weak and (relatively) strong syllables. These
famous lines of Shakespeare's illustrate how it works:

Accent is purely relative. Syllables like "let", "not", and "of" are very weak,
but they count as accents when the syllables around them are even weaker. Conversely, "true"
is a strongly stressed syllable, but it counts as unaccented because the next syllable,
"minds", is even stronger.

The arrangement of accented and unaccented syllables falls into a regular pattern,
with only occasional exceptions. In this case the pattern is what in traditional poetry
is called iambic: an even alternation of one unaccented followed by one accented syllable.

The number of accents stays the same from line to line: in this case, there are five accents
per line, so that the meter is what is traditionally called iambic pentameter.

The usual way of analysing traditional metrical poetry is to divide the line into feet which
mark the regular alternation of accented and unaccented syllables. For example, the first line of
the quote from Shakespeare can be divided into five feet, like this:

Notice at once several things which distinguish alliterative verse from traditional metrical
verse:

We do not have a regular alternation between accented and unaccented syllables. The strong
syllables can appear back to back, or separated by one, two or (potentially) even more weak
syllables. The number and arrangement of weak syllables is variable (though not arbitrary,
as we shall see.)

The strong syllables are naturally strong syllables; the weak syllables are naturally weak
syllables. We do not as a general rule see weak words like "of" or "to" treated as if they
bore emphasis.

The number of strong syllables per half line is strictly regulated: in general, as in this case,
there are two.

The pattern of alliteration only applies to the strong syllables.

We need terms for the strong and weak elements in alliterative verse so we will not confuse them
with the accented and unaccented elements in traditional metrical verse. So let us adopt the following
terms (traditionally used in some scholarly circles) --

a lift is one of the strongly stressed parts of an alliterative line, while
a dip is one of the weak parts of an alliterative line.

All the rules for alliterative verse are rules for lifts, dips, and how they can be arranged. In sections
to follow we will explore some of these rules.

Note: Some scholars use "thesis" instead of "lift" and "arsis" instead of "dip", especially when
talking about Anglo-Saxon poems like Beowulf. I will restrict myself to "lift" and "dip" except when discussing
Anglo-Saxon alliterative verse, which has its own special rules.